Thrombin receptors define responsiveness of cholesterol-modified platelets

The microviscosity of human platelet membranes was changed by incubating platelets with liposomes containing various ratios of cholesterol and lecithin. Binding of 125I-thrombin to the modified platelets was measured together with platelet aggregation and secretion. In cholesterol-normal platelets (mole ratio of cholesterol to phospholipid (C:PL) = 0.553; eta = 2.40 poise), weighted nonlinear least squares curve fitting indicated that a model involving two classes of sites was adequate to describe the binding isotherm (K1 = 8.3 X 10(8) M-1; R1 = 150 sites/platelet; K2 = 6.4 X 10(6) M-1; R2 = 16,000 sites/platelet). In cholesterol-enriched platelets (C:PL = 0.857; eta = 3.05 poise), the apparent affinities for the two classes of sites decreased to 55 and 53%, respectively, while the binding capacities increased to 170 and 160%, respectively. In contrast, in the cholesterol-depleted platelets (C:PL = 0.435; eta = 2.03 poise), the affinities increased to 220 and 180%, respectively, while the binding capacities decreased to 53 and 46%, respectively. In cholesterol-enriched, cholesterol-normal, and cholesterol-depleted platelets, the thrombin concentrations required for half-maximal aggregation were 0.17, 0.35, and 0.52 nM, respectively, while the values for half-maximal secretion of [14C]serotonin were 0.17, 0.40, and 0.55 nM, respectively. Plots of receptor occupancy versus biological response showed that maximum response in cholesterol-enriched, cholesterol-normal, and cholesterol-depleted platelets occurred with occupancy of 30, 50, and 70% of the high affinity sites, respectively. In all three treatment groups, occupancy of 40-50 high affinity sites results in 50% aggregation. These results show that (i) modification of platelet membrane microviscosity results in changes in the number and affinity of both high and low affinity thrombin receptors, (ii) the change in receptor number rather than affinity is the determinant for platelet responsiveness, and (iii) the changes in membrane microviscosity do not appear to alter the coupling between occupied receptor and subsequent bioresponse.